Blood-vessels of nerve are comparable to those of the central nervous system, and are lined by a continuous endothelium with intercellular tight junctions. They form part of the blood-nerve barrier, together with the perineurium. The electrical properties of the perineurium were examined by AC impedance techniques, which demonstrated that the perineurium has a high resistance to ion flow and electrical polarization characteristics of intercellular tight junctions. Hypertension, induced by vascular perfusion of the nerve damages the vascular endothelium by forming submembrane blisters, and increases permeability to intravascular tracers, horseradish peroxidase and microperoxidase. Alkaline phosphatase, an enzyme characteristic of epithelia involved in transport, is found in blood vessels of the rat but not of the frog endoneurium, and in the perineurium of the frog nerve but not of the rat nerve, suggesting differential transport properties of the tissues in relation to species. Low permeabilities of the frog sciatic nerve suggest that ions cross the capillaries by the paracellular route, and are not transported actively across the vascular endothelium. Similarly, calcium appears to traverse the frog perineurium by a passive diffusion. Capillaries of the frog sciatic nerve demonstrate increased permeability following nerve transection and after 1 week of Wallerian degeneration. Endoneurial capillary integrity therefore depends on the integrity of the nerve itself. An adequate blood supply to the nerve is required to maintain nerve function and integrity. Blood flow in the rat sciatic nerve was measured with a laser Doppler flowmeter and radiotracer techniques, and was shown to be close to that found for white matter of the brain. During systemic hypotension, there does not appear to be autoregulation of flow, as flow falls in proportion to systemic blood pressure.